In a previous papcrl it was shown that, under suitable conditions, a yellow dioxanc solution of z,z',4,4',G,G'-hexanitrodiphenylaminc (= HNDA) was turned orange or red on the addition of pyridine through the dissociation of the acid, giving red anions. This reaction is sufficiently scnsitivc tc5 permit calorimetric dctcrmination of pyridine.
Pyridinc is gcncrally dctcrmincd by direct titration with standard acid or by back-titration with alkali in aqueous solution using methyl orange as the indicator. A volumetric determination of pyridine in non-aclucous media was suggested by PALIT~, in a mixture of solvents containing glycol and higher alcohols or chloroform, and by FRITZ" using bcnzcne, chlorobenzene. nitrobenzenc, chloroform or petroleum cthcr as solvent and titrating with perchloric acid in glacial acetic acid. 130th these authors used either the potentiomctric method or indicators. In a similar environment a high frequency oscillometer proved suitable for the determination of pyridin@. Pyridinc has been estimated gravimetrically as its chlorauratc, its chloroplatinate ancl as its pcrchloratc 610. For a colorimctric determination of pyridine various reactions have been proposed: the reaction with cyanogen bromide producing the red colour of glutaconic aldehyde '; the reaction with cyanogcn bromide and benzidine hydrochloridee ; the conversion of pyridine into qpyridyl-pyridinium dichloride9 ; colour tests with sodium in ethanol in a chloroform-water mixture'"; the Fujiwara rcactionll and the reaction with barbituric acid 12. Pyridine was further determined spectrophotometrically in the ultraviolet region either after extraction with dilute phosphoric acid13 or directly14. Infrared spectroscopy18 as well as chromatography16 have also been suggested for the quantitative analysis of pyriclinc. 'rho I-INI>A wi\S an Enstman Kodak rcagcnt for laboratory USC ant1 \viLs not rccrystallizcd. The dioxanc, the pyridinc z\nd absolute ethyl alcol~ol wcrc 13.D.H. redistillccl rcagcnts. o. I IL1 and 0.01 M solutions of HNDA were prcparcd by dissolving the yellow crystals in dioxanc. lk2sh solutions wcrc always usccl, since tlicsc solutions aro unstable, turning appreciably orange in 24 hours. For tlic prcpnration of the cdibration ciirvc o.os"k, w/v and o.~O/~ w/v solutions of pyriclinc in clioxanc wcro used, and for the clctcrmination of pyriclinc in ethyl alcohol (dcnaturccl alcohol) a 0.50/~ v/v pyriclino in 95% ethyl alcohol solution was usccl. References p. x57